Fix sending wxEVT_TEXT_ENTER when using auto-completion in wxMSW

We need to explicitly generate this event from the char hook handler as
we don't get the normal WM_CHAR for it, it is apparently intercepted by
the window proc installed by the auto-completing code, so check if
wxTE_PROCESS_ENTER is used for the text entry and call a special new
MSWProcessSpecialKey() method to do the right thing if it is.

Similarly, handle Tab presses correctly if wxTE_PROCESS_TAB is used.

Closes #12613.

include/wx/msw/combobox.h

@@ -158,6 +158,14 @@ private:
     virtual wxWindow *GetEditableWindow() wxOVERRIDE;
     virtual WXHWND GetEditHWND() const wxOVERRIDE;
 
+    // Common part of MSWProcessEditMsg() and MSWProcessSpecialKey(), return
+    // true if the key was processed.
+    bool MSWProcessEditSpecialKey(WXWPARAM vkey);
+
+#if wxUSE_OLE
+    virtual void MSWProcessSpecialKey(wxKeyEvent& event) wxOVERRIDE;
+#endif // wxUSE_OLE
+
     // common part of all ctors
     void Init()
     {